Equalization of superheated and reheated steam temperature in steam power plants



Aug; 3, 1954 v. z. CARACRISTI 2,685,279

EQUALIZATION 0F SUPERHEATED AND REHEATED STEAM TEMPERATURE IN STEAMPOWER PLANTS 10, 1952 4 Sheets-Sheet 1 Filed Jan.

yflef Reheaterlnlet I Superheuter 221 ,231 Inlet 3/ l7 Reheater 0Generator INVENTOR r C m Y PU MW Z R .m w M W Y, g F m n e d n O C .m

g- 1954 v. z. CARACRISTI 2,685,279

EQUALIZATION OF SUPERHEATED AND REHEATED STEAM TEMPERATURE IN STEAMPOWER PLANTS Filed Jan. 10, 1952 4 Sheets-Sheet 2 INVENTOR 'Virginius Z.Corocrisfi ATTORNEY Aug. 3, 1954 v. z. CARACRISTI 2,685,279

EQUALIZATION 0F SUPERHEATED AND REHEATED STEAM TEMPERATURE IN STEAMPOWER PLANTS Fxled Jan 10, 1952 4 Sheets-Sheet 3 Superheater M M A m m mmu m mm vQv 3B N 4 n m a 8 9 .m M u 0 m r & e 0 m 9 4 3 4 I 4 u h 9 9 m8 m z G 4 u a r I I I I I I l I I I I I I I I I I I I I I I 9 I M F ToCondenser INVENTOR ATTORNEY Aug. 3, 1954 v. z. CARACRISTI 2,685,279

EQUALIZATION OF SUPERHEATED AND REHEATED STEAM TEMPERATURE IN STEAMPOWER PLANTS Flled Jan 10, 1952 4 Sheets-Sheet 4 Fig. 4;

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Generator To Condenser ATTORNEY Patented Aug. 3, 1954 EQUALIZATION OFSUPERHEATED AND RE- HEATED STEAM TEMPERATURE IN STEAM POWER PLANTSVirginius Z. Caracristi, Bronxville, N. Y., assignor to CombustionEngineering, Inc., New York, N. Y., a corporation of DelawareApplication January 10, 1952, Serial No. 265,777 14 Claims. (01. 122479)This invention relatesto vapor power plants in which superheated vaporafter having given up part of its energy is reheated in a reheater. Morespecifically this invention-relates to steam power plants operatingunder the reheat cycle.

In the operation of modern high pressure, high temperature steam powerplants it is essential that the temperature of-the steam entering thesteam turbine be held at a constant value over a wide range of load.

Moreover, in a steam powerplantoperating under the reheat cycle itbecomes necessary to control not only the primary superheated steamtemperature but also the reheated steam temperature and likewise tomaintain it at a constant value over a wide range of load. The operatorof a steam generator of this reheat'type is accordingly confronted withcontrolling at the same time both the heat transfer to the superheatersurface and to the reheater surface by changing either the temperatureof the combustion gases flowing over the aforesaid heating surface, orthe quantity thereof, or both.

* It hasbeen found in the operation of a-steam generating. power plantof the type--herein consideredthat at low loads the-superheater absorbsmore heat than is required tomaintain-thedesired superheatedsteamtemperature if=--the reheat temperature is maintained atthe-desired value and that in these cases it becomes necessary to employauxiliary control equipment,-such as for example a desuperheater, tolower the temperature of the superheated steam to "the design valuebefore enteringthe turbineyand utilization in the boiler furnace of asteam reheater further complicatesthe control problem by requiringsupplemental means (again such as a desuperheater) for keeping thereheated-"and superheated steam temperatures at predetermined values asthe turbine loading (and hence steam generator loading) varies.

Methods of obtaining equalization of the superheated steam and reheatedsteam temperatures as theone hereinabove described, in which a--desuperheater is utilized, are not only costly to maintain but alsolower the overall plant efficiency.

, It is accordingly the main object of myinvention to provide a superiormethod of temperature equalization between the superheated steamtemperature and the reheated steam temperature. Another object of myinvention is to: provide for effective protection of the reheaterheatingsurfaceduring starting up. periods or during periodsavhe'n little ornoreheat steam flows such as during a trip-out when the reheat interceptvalve closes.

Still another object of the invention is to permit a higher heat releaserate in the furnace than was heretofore possible.

A further object of my invention is to permit a wider operating range ofprimary and reheat steam temperature control.

Another object of the invention is to lower the design pressure lossthrough the reheater.

Other objects and advantages of the invention will become apparent fromthe following description of illustrative embodiments thereof when takenin conjunction with the accompanying drawings wherein:

Fig. l is a diagrammatic representation of a power plant operating underthe reheat cycle showing my inventive improvement including a steamgenerator equipped with tilting burner control.

Figs. la-lb-lc indicate in diagrammatic view how the tilting burners ofthe Fig. l furnace may be adjusted respectively to locate the burningfuel flame at raised, intermediate and lowered positions in the furnacecombustion chamber.

Fig. 2 is a diagrammatic representation of a power plant operating underthe reheat cycle incorporating my inventive improvement and including asteam generator equipped with gas recirculation temperature control.

Fig. 3 is a diagrammatic representation of a power plant operating underthe reheat cycle, equipped with my inventive improvement and showingasteam generator in which steam temperature control is accomplished bymeans of a gas by-pass and/ or a desuperheater.

Fig. 4 constitutes a plan section taken on line 4-4 of each of Figs. 1,2 and 3 and showing my new arrangement of superheater and reheater tubeswithin the furnaces upper portion.

Fig. 5 is a plan section taken on line 5-5 of Fig. 2 showing cornermounting of the furnace burners for tangential firing.

Fig. 6 is a plan section taken on line 6-5 of each of Figs. 1, 2 and 3showing one novel feature of my invention in an enlarged scale.

Fig. '7 is an elevational section taken on line l-'l of Fig. 1 showingfurther details of the new superheater-reheater assemblage.

Fig. 8 is a plan section taken on line 88 of Fig. 1 showing anotherfeature of my inventive improvement in an enlarged scale.

In the illustrative embodiment of my invention herein disclosed, a steamgenerator generally designatedby A comprises a furnace I the walls ofwhich are lined with steam generating tubes (not shown) in a manner wellknown in the art. These tubes discharge the steam generated therein intodrum 2 from which the steam leaves through tubes 3 connecting the drumwith the superheater inlet header 4. There are two such headers shown inthe steam generator described.

Burners 5 deliver fuel entrained in a stream of air for combustion intothe furnace l. The fuel burns within the furnace: and the ash falls intothe hopper bottom 8 from which it is removed by conventional means (notshown) The products of combustion rise within the furnace and pass overheating surfaces representing my inventive superheater and reheatercombination, generally designated B. The gases then enter a lateral pass7 from which they are discharged into a downward pass 8 whereinadditional banks of heat absorbing tubes are located represented in thisdescriptive illustration by low temperature superheater section 9. Uponleaving low temperature superheater section 9 the gases may pass overother heat recovering surfaces before being discharged into the stack(not shown).

The saturated steam having entered superheater headers 4 as hereinabovedescribed flows through the above mentioned low temperature section 5wherein an initial degree of superheat is imparted thereto. Outletheader is and pipe ll serve to conduct this partially superheated steamto the inlet header l2 of the high tem perature superheater section Bswhich forms a part of the hereinabove mentioned superheater and reheatercombination B. In order to visually distinguish the superheater Bs fromthe later herein described reheater ET. the superheater is shown insolid lines whereas the reheater is shown in dotted linesv The steamhaving been superheated to a temperature corresponding to the turbinedesign vaue by passing through successive U-shaped tubes of superheaterBs and absorbing heat from the combustion gases, thereupon leaves by wayof superheater outlet header 13 from where it is conducted through pipeit to the high pressure stage of turbine Hi. In the arrangement hereshown this turbine drives an electric generator G.

The steam having passed through some of the high pressure stages ofturbine l5 and having given up apart of its energy is returned to thesteam generator by way of pipe line it? for reheating. For this purposethe steam enters the inlet header 5! of the reheater Br forming anintegral part of my inventive superheater and reheater combination B ashereinafter described. After being reheated to the proper turbine designtemperature by absorbing heat from the gases flowing over the reheatersurface Br (shown in dotted lines) the steam leaves by way of reheateroutlet header i3 and is returned through pipe I 9 to an intermediatepressure stage of turbine l5 wherein the remaining heat in the steam isconverted into mechanical energy. The steam then leaves turbine if: tobe condensed in a condenser (not shown).

Burners 5 of the steam power plant illustrated in Fig. 1 are of thetilting type, and are preferably located adjacent the four corners ofthe furnace as illustarted in Fig. 5. Preferably these burners 5 directthe fuel and air into the furnace in streams tangential to an imaginaryvertical cylinder within the furnace as indicated in Fig. 5. Howeverother burner arrangements of course are usable to deliver fuel and airinto the furnace i for combustion therein. The preferred tilting burnerconstruction illustrated in Fig. 1 permits direction of the fuel and airstreams upwardly towards the superheater and reheater heating surface Bor downwardly away from the superheater and reheater heating surface B;such actions being shown in diagrammatic form by Figs. la, 11) and 10.

By tilting the fuel and air streams in this manner, which is well knownin the art, the tem perature of the gases entering heating surfaceslocated above the furnace proper, such as superheater and reheatercombination B, can be controlled to permit raising or lowering of thesuperheated and reheated steam temperature. This control is exceedinglyimportant in the operation of modern high temperature, high pressuresteam generators because it is essential for the efficient operation ofthe steam turbine to deliver thereto steam at a previously detenmineddesign temperature.

As hereinabove set forth the difficulties arising in the operation of amodern reheat power plant are aggravated by the necessity toindependently regulate the steam temperature of both the reheated steamas well as the primary superheated steam. In many installations this isaccomplished by the use of a desuperheater located between the high andlow temperature superheater sections such as for example, in steam lineH or in the steam pipe l4 leading to the turbine 15.

My invention, providing for an integrally combined superheater andreheater. eliminates the use of supplementary auxiliary control devicessuch as a desuperheater. This I accomplish by constructing areheater-superheater organization in the form of panels or platens Bwhich are relatively widely spaced such as indicated in Fig. 4. Theseplatens comprise a plurality of tubes 28 and 21 which are arranged sideby side and welded one to the other to maintain intimate metal contact.This is shown in Fig. 4 and in an enlarged scale in Figs. 6 and 8.

In my inventive combination superheated steam entering inlet header l2flows through tube 20 to absorb heat from the combustion gases byradiation as well as by convection. The reheat steam returning from theturbine for reheating enters reheater inlet header H and flows throughreheater tube 2! to also absorb heat from the combustion gases byradiation and convection.

These tubes 20 and 2| can be arranged in any convenient manner such asthe here represented groups of nested U-shaped tubes forming panels Band providing within each panel parallel flow, as shown in Figs. 1, 2,and 3; each panel being serially connected to the adjacent one byconnecting tubes 3!. Even though vertical runs of tubes 20 and 2| arehere shown, it will be understood that these superheater and reheatertubes also can be arranged in horizontal runs.

Moreover, the tubes in and 2! can also be formed into continuous coilsfor pure serial flow, one coil carrying superheated steam and anadjacent coil carrying reheated steam, both maintaining intimate metalcontact substantially throughout their heat exposed length. Eitherparallel or serial flow can be provided; however the direction of flowof the superheated steam and that of the reheated steam preferablyshould be the same. 7

The enlarged cross section shown in Fig. 8 indicates the intimatecontact which is maintained between superheater tube 20' and reheatertube 2!. This metal-to-metal contact causes heat transfer to take placebetween the fluid flowmg through tube 20 and that flowing through tube2| if a temperature difference exists between the two fluids. In mostcases it is desirable to provide the turbine with superheated steam andreheated steam having equal temperatures. However a substantialdifference of temperature usually exists between the saturated steamleaving drum 2 and entering superheater inlet header [2 and the reheatsteam leaving an intermediate pressure stage of turbine l5 and enteringreheat inlet header I1.

My invention accomplishes equalization of these reheated and superheatedsteam temperatures. Such equalization takes place because thesuperheater tubes 20 maintain intimate metal contact with reheater tubes2| substantially throughout their entire heat exposed length; hence thetemperature difference that may exist at the inlet headers 12 and Hissubstantially if not entirely eliminated by the time the superheated andreheated steam has reached the outlet headers l3 and iii.

For example, if we consider a reheat generator operating under a steampressure of approximately 1800 lbs. per sq. in., the temperature of thesteam entering the superheater header !2 may be in the neighborhood of600 deg. F. The temperature of the steam entering the reheater header i!on the other hand may, at low loads, be in the neighborhood of 400 deg.F., whereas at maximum load the temperature of the steam entering headerI! to be reheated may reach a value as high as 700 deg. F. The finaltemperature leaving the superheater as well as that leaving the reheatermay be in the neighborhood of 1000 deg. F. There are of course certaincases in which it is desirable for reasons of economy in steam turbinedesign to increase the temperature of the reheated steam over that ofthe primary superheated steam because the reheated steam is delivered tothe turbine under a much lower pressure than that of the primarysuperheated steam.

It can well be appreciated by those skilled in the art of designingmodern steam power plants, how my invention permit simplification of thecontrol problem with which steam power plant designers are faced. Thiswill become apparent when it is considered that my inventive improvementcauses equalization of the superheated steam temperature and thereheated steam temperature regardless of whether the superheated steamtemperature entering my inventive superheater-reheater B is higher thanthe entering reheat steam temperature, or lower. In both cases theintimate metal contact afforded between the superheater tube and thereheater tube, as shown in Figs. 6 and 8, will accomplish heat transferbetween steam flowing through the superheater tube 20 and that flowingthrough the reheater tube 2|. If it should become desirable ashereinabove set forth to furnish the turbine with reheated steam havinga temperature somewhat higher than the superheated steam, this can quiteeasily be accomplished by extending the heating surface of the reheaterthrough lengthening the lower end of the reheater tubes beyond thelength of the superheater tubes. My invention therefore can be appliedto any temperature conditions that may be required for obtaining amaximum overall power plant eiiiciency.

Fig. 7 shows an elevational section of the upper end of two parallelsuperheater-reheater platens or panels, each platen formed of U-shapedtubes 20, 2 I terminating at its upper end into four headiii) ers, twosuperheater headers-an inlet header 22a and an outlet header 22b, andtwo reheater headersan inlet header 23a and an outlet header 23b. Thereare three tube banks 24 shown in Fig. 1 comprising spacedly arrangedU-shaped platen sections B. These platens in forming the said tube banksmay satisfactorily be spaced at distances approximately two feet acrossthe width of the unit, as shown in Figs. 4 and 7. To allow for expansionbetween the individual U-shaped platens, guiding brackets 25 are weldedto one section and extend over the width thereof to form a shoe aroundthe adjacent platen section for guidance thereof as shown in Figs. 6 and8. This will allow each section to expand independently of the other,yet retain the adjoining sections in alignment. To maintain the verticalsides of the platen B in alignment portions 28 oi furnace wall tubes maybe bent out to form a guide channel within which the platens can expandas shown in Fig. 6. Other means well known in the art for supporting andguiding these platen sections or panels 24 of course can also beemployed.

My invention as herein disclosed contemplates the use of a controllingdevice for regulating the temperature and/or quantity of the gasesentering superheater and reheater surfaces B tomaintain constant steamtemperature for various steam loads. The necessity of such a control forattaining constant superheated steam tempera ture over a given controlrange is widely recognized and various means are employed for thispurpose, such as burner tilting as illustrated in Fig. 1 or gasrecirculation as shown in Fig. 2.

Operation of the burner tilting facilities of Fig. 1 will best beunderstood through reference to Figs. la-lb-lc. If the temperature ofthe steam leaving superheater-reheater assemblage B is too low, theflame center can be raised as shown by Fig. 1a; and if the temperatureis too high the flame center can be lowered as shown by Fig. 10. Suchadjustments can'be effected automatically through the medium of burnertilting motor 32 as later described.

In the gas recirculation arrangement of Fig. 2 relatively cool gas fromofftake 21 is forced through duct 26, as by means of fan 44, into thefurnace i wherein it mingles with the combustion gases before flowingover superheater-reheater surface B; control of such recirculation beingeifected by means of a damper 42 or in some other way as laterexplained. My invention thus can be used in combination with a gasrecirculation control equally well as With tilting burner control; sincewith either arrangement the reheated steam temperature and thesuperheated steam temperature can in my inventive combination be treatedas one and control of either the super-- heated steam or the reheatedsteam by means of gas recirculation will at the same time control theother by virtue of the'heat transfer taking place between welded tubes29 and 2!.

In addition to the two control arrangements discussed hereinabove inconnection with my invention, namely tilting burners and gas recirculation, it is also possible to control the heat absorption of thesuperheater or indirectly that or" the reheater by by-passing gasesaround the low temperature section of the superheater as indicated inFig. 3; an adjustable damper it then being utilized as later explained.

It is also possible in a steam generator equipped with my inventiveimprovement to control the temperature of the reheat steam entering thereheater inlet header H and thereby indirectly the temperature of thesuperheated steam, by installing a desuperheater 30 in the steam line itbetween the turbine 15 and the inlet header I! as shown in Fig. 3.

Automatic control can also be provided for my inventive combination whenusing any one of the four above mentioned main control arrangements.Thus in Fig. 1 a burner-tilting motor 32 is shown the action of which iscontrolled through suitable means 25 by the temperature actuated element33 which responds to temperature changes of the superheated steamleaving superheater outlet header [3 (if desired element 53 can insteadbe placed in reheater outlet header IS). The illustrated motor 32 isarranged by means of gearing 34 and linkage 35 to raise or lower thefuel and air ejecting nozzles 36 of tilting burners 5 (preferably all ofthem in parallel) thereby accomplishing the raising or lowering of themass of burning gases with in the furnace proper as diagrammaticallyillustrated by Figs. la-lb-lc. A change of combustion gas temperature isthereby accomplished as hereinabove set forth.

Similarly the recirculation of gases for furnace outlet temperaturecontrol as shown in Fig. 2 can be controlled by automatic meanscomprising a motor 38, the action of which is governed through controlmeans 39 by temperature changes of the steam leaving either superheateroutlet header 13 (wherein element 33 is here shown as being disposed) orreheater outlet header 18 (wherein element 33 may instead be disposed).The illustrated motor 33 drives a reduction gear so which in turnactuates damper 42 located in gas recirculating duct 26. It is of coursealso possible to provide for automatic control of gas recirculation bydirectly regulating the speed of fan M which in turn determines the fiowof cool gases from ofitake 21 into furnace i.

In many cases it is also desirable to provide automatic control forregulating the flow of gases over low temperature superheater section 9by a control such as indicated in Fig. 3 wherein a temperature controldevice 48 responsive to the temperature registered by element 33(disposed either in reheater outlet header i8 or in superheater outletheader [3 as shown) actuates motor 5% driving gear 52 which opens orcloses damper 54. Such automatic control of damper 5:5 is effective aslong as transfer switch 58 occupies the right position shown by Fig. 3.

Similarly the flow of spray water utilized in desuperheater 36 can beautomatically controlled by means of motor 58 driving gearing 53 i whichin turn opens or closes valve 60 located in water supply pipe 62delivering spray water to the desuperheater from a source (not shown).Such automatic control of desuperheater 3G is effective when transferswitch. 59 is shifted to the left (from the right position shown by Fig.3) thereby connecting temperature registering element 33 (in eithersuperheater outlet header 13 or reheater outlet header 18) with means 63which translate changes in the leaving steam temperature intoappropriate adjusting operations by motor 56.

While illustrative embodiments of my invention have been here shown anddescribed, it will be understood that changes in construction,combination, and arrangement of parts may be made without departing fromthe spirit and scope of the invention as claimed.

I claim:

1. In a steam generator of the type described adapted to operate atdifferent loads, a boiler having a plurality of heat absorbing tubespositioned therein, a first group of said tubes comprising asuperheater, a second group of said tubes comprising reheater, therelative heat absorption requirements of the superheater and reheaterbeing different at difierent loads, the tubes of said first and secondgroup being positioned in defined rows wherein adjacent tubes areassociated with a different group, header means associated with thetubes in each of said rows effective to direct the fiow of steamserially through the tubes that comprise the superheater in each of saidrows and similarly through the tubes that comprise the reheater in eachof said rows, the tubes in said defined rows being positioned in directheat exchange relation with one another whereby the temperatures of thesteam within said superheater and within said reheater tend to equalize.

2. In a vapor generator of the type described operating on the reheatcycle and over a substantial load range, the combination of a boilerhaving a plurality of metallic heat absorbing tubes positioned therein,a first group of said tubes comprising a superheater with the tubesthereof disposed in defined spaced rows, a second group of said tubescomprising a reheater with the tubes thereof in parallel relation withand intercalated among the tubes of said superheater, the boiler,including the superheater and reheater being arranged to be heated bythe same heat source, said tubes being arranged so that the vapor flowsin the same direction through the tubes associated with each of saidgroups, means bonding the tubes in said rows together in metal to metalrelationship thereby providing for good exchange of heat between saidsuperheater and said reheater.

3. In a vapor generator of the type described adapted to operate over awide range of loads, a boiler having a plurality of metallic heatabsorbing tubes positioned therein, a first group of said tubescomprising a superheater with the tubes thereof forming defined spacedrows, the tubes in each of said rows being spaced predeterminedintervals from each other, a second group of tubes comprising a reheaterwith the tubes thereof positioned between the tubes of said first groupand substantially filling said predetermined intervals therebetween, therelative heat absorption requirements of the superheater and reheaterbeing diiierent at difierent loads, said superheater and reheater tubesbeing arranged so that the flow therethrough is in the same direction,means securely bonding together in metal to metal relationship adjacenttubes in each of said rows thereby providing for a good exchange of heatbetween adjacent tubes.

4. A steam generator operating on the reheat cycle and over a wide loadrange, said generator comprising a generally vertical boiler havingburner means disposed at one end thereof and provided with a pluralityof vertically disposed horizontally spaced rows of tubes adjacent theother end, adjacent tubes in each of said rows being physically unitedthroughout at least a major portion oi their length, alternate tubes ineach of said rows forming the superheater for said steam generator andthe remaining tubes in each of said rows forming the reheater for saidsteam generator, header means for said tubes constructed and arranged soas to provide for serial flow through said superheater and said reheatertubes in each row and further to provide for the flow through saidsuperheater and said reheater tubes in the same direction.

5. A steam generator operating on the reheat cycle and over asubstantial load range, said generator including a gas pass throughwhich the hot combustion gases flow, a plurality of vertically disposedhorizontally spaced rows of metallic tubes positioned in said gas passto absorb heat from the hot gases of combustion flowing therethrough,said rows being parallel with the gas flow and comprising numerousU-shaped tubes nested within one another and united in metal to metalrelation to form a rigid substantially flat panel, alternate tubes ineach of said rows being interconnected in such a manner as to form thesuperheater for said steam generator with the remaining tubes in each ofsaid rows being interconnected in such a manner as to form the reheaterfor said steam generating means, with the direction of flow throughadjacent tubes being the same.

6. In a system of the type described, a steam generator operating on thereheat cycle and at substantially different loads, said generator havinga steam superheater means and a, steam reheater means disposed therein,the generator including the superheater means and reheater means beingarranged to be heated by the same source of heat, control meansresponsive to the temperature of the steam leaving one of theaforementioned means and operable to maintain said temperaturesubstantially constant, said superheater means comprising a plurality ofspaced metallic tubes communicating with superheater inlet and outletheaders, said reheater means comprising a plurality of metallic tubesintercalated among and parallel with said superheater tubes, saidreheater tubes being bonded in metal to metal relation with adjacentsuperheater tubes and communicating with suitable inlet and outletheaders whereby said superheater means and said reheater means are inheat exchange relationship with one another resulting in automaticallyre ulating within limits the temperature of the steam at the outlet ofboth by a single control means.

'7. In a steam generator operating on the reheat cycle and over asubstantial load range, said generator includin a furnace, means forsupplying fuel to and burning it in said furnace and steam generatingsurface absorbing heat from the burning fuel and from the gases ofcombustion; steam heating elements disposed in the path of the gasesdischarged from the furnace comprising a panel of heat absorbingelements made up of metallic tubes disposed in side by side relation inrows spaced transversely of the gas path; means for regulating the heatexchange relationship between the steam flowing through said tubes andthe stream of gases contacting the latter; means including headersconnected so as to direct a stream of steam from said generator throughcertain tubes in said panel and for conducting the superheated steam toa steam user; and means including header means for directing steam fromsaid user through other tubes in said panel adjacent to said certaintubes and in the same direction with respect to the flow of gasesthereover as the steam flowing through said certain elements, the tubesof said panel being in direct heat exchange relationship with each otherto equalize the temperature of steam heated therein by contact with themass of gases flowing thereover.

8. In a steam generator operating on the reheat cycle and over asubstantial load range, said generator having a furnace, means forsupplying fuel to and burning it in said furnace, and steam generatingsurface extending generally throughout the length of the furnace forabsorbing heat from the burning fuel and from the gases of combustion;steam heating elements disposed in the path of the gases discharged fromthe furnace comprising a plurality of parallel panels of heat absorbingelements each made up of metallic tubes disposed in side by siderelation in rows spaced transversely of said gas path; means forregulating the differential between the temperature of steam flowingthrough said tubular elements and the temperature of the mass of gasescontacting said elements consisting of means for moving the zone ofcombustion of fuel in the furnace toward. or away from its outlet toalter the amount of heat absorbed by said steam generating tubestherein; means including headers connected so as to direct a stream ofsteam from said generator in series relation through alternate tubes insaid panel and for conducting the superheated steam to a steam user; andmeans including header means for directing steam from said user throughthe intermediate tubes in said panel in series relation and in the samedirection with respect to the flow of gases thereover as the steamflowing through said alternate elements, said alternate and intermediatetubes of each panel being in direct heat exchange relationship with eachother throughout a major portion of their length to substantiallyequalize the temperature of steam heated therein.

9. In a steam generator operating on the reheat cycle and over asubstantial load range, said generator including a furnace, means forsupplying fuel to and burning it in said furnace and steam generatingsurface absorbing heat from the burning fuel and from the gases ofcombustion; steam heating elements disposed in the path of the asesdischarged from the furnace and comprising a plurality of parallelpanels of heat absorbing elements made up of metallic tubes disposed inside by side relation in rows spaced transversely of said gas path;means for regulating the heat exchange relationship between the steamflowing through said tubular elements and the stream of gases contactingsaid elements comprising means for controllably returning to the furnacea volume of gases after they have passed in contact with said steamgenerating and steam heating means; means including headers connected soas to direct a stream of steam from said generator through certain tubesin said panel and for conducting the superheated steam to a steam user;and means including header means for directing steam from said userthrough tubes in said panel adjacent said certain tubes and in the samedirection with respect to the now of gases thereover as the steamflowing through said certain tubes, said certain and other tubes of eachpanel being in direct heat exchange relationship with each other tosubstantially equalize the temperature of steam heated therein.

10. Inv a steam generator operating on the reheat cycle and over asubstantial load range, said generator including a furnace, means forsupplying fuel to and burning it in said furnace and steam generatingsurface absorbing heat from the burning fuel and from the gases ofcombustion; steam heating elements disposed in the path of the gasesdischarged from the furnace and comprising a plurality of parallelpanels of heat absorbing elements each made up of metallic tubesdisposed in side by side relation in rows spaced transversely of saidgas path;

means for regulating the heat exchange relationship between the steamflowing through said tubular elements and the stream of gases contactingsaid elements comprising a low temperature superheating section disposedbeyond said steam heating means in the path of gas flow from the furnaceand a damper control by-pass for apportioning the fiow of gases oversaid primary superheater and through said by-pass to effect saidtemperature regulation; means including headers connected so as todirect a stream of steam from said generator through certain tubes insaid panel and for conducting the superheated steam to a steam user; andmeans including header means for directing steam from said user throughtubes in said panel adjacent said certain tubes and in the samedirection with respect to the fiow of gases thereover as the steamflowing through said certain tubes, said certain and other tubes of eachpanel being in direct heat exchange relationship with each other tosubstantially equalize the temperature of steam heated therein.

11. In a steam generator operating on the reheat cycle and over asubstantial load range, said generator including a furnace, means forsupplying fuel to and burning it in said furnace and steam generatingsurface absorbing heat from the burning fuel and from the gases ofcombustion; steam heating elements disposed in the path of the gasesdischarged from the furnace and comprising a plurality of panels of heatabsorbing elements each made up of metallic tubes disposed in side byside relation in rows in said gas path; means including headersconnected so as to direct a stream of steam from said generator throughcertain tubes in said panel and for conducting the superheated steam toa steam user; means including header means for directing steam from saiduser through other tubes in said panel adjacent to said certain tubesand in the same direction with respect to the fiow of gases thereover asthe steam flowing through said certain elements, said tubes of eachpanel being in direct heat exchange relationship with each other toequalize the temperature of steam heated therein by contact with themass of gases flowing thereover; and desuperheating means interposedbetween the outlet of said superheater and the inlet of said reheater.

12. A steam generator operating over a wide load range and comprising aboiler provided with steam generating means, means for superheating thesteam produced by said steam generating means, said superheating meansincluding a first heating conduit for how therethrough of the steam tobe superheated, a steam reheating means including a second conduit forflow therethrough of the steam to be reheated, the relative heatabsorption requirements of the superheater and reheater being differentat different loads, said first and second conduits being physicallyunited throughout at least a substantial portion of their lengths withthese imited conduit porions arranged so the steam flow therethrough isin the same direction.

13. A vapor generator adapted to operate at substantially differentloads and having a plurality of heat absorbing tubes positioned therein,a first group of said tubes comprising a superheater, a second group ofsaid tubes comprising a reheater, the relative heat absorbingrequirements of said superheater and reheater being different atdifierent loads, at least a major portion of the tubes of said first andsecond group being arranged so that the direction of vapor fiowtherethrough is the same, with these portions of the tubes of each groupbeing in direct heat exchange relation with each other.

14. A steam generator operating on a reheat cycle and over a substantialload range, comprising a boiler provided with steam generating means,means for superheating the steam produced by said steam generatingmeans, a steam reheating means, said generator means, superheating meansand reheating means being heated by the same source of heat, asubstantial portion oi said superheating means and said reheating meansbeing arranged in direct heat exchange relation with one another withthese portions having steam flow in the same direction so that thetemperature at the outlet of said superheater means and said reheatermeans will tend to be equal.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 258,933 Litchfield et a1 June 6, 1882 1,952,542 Ehlinger Mar.27, 1934 2,602,433 Kuppenheimer July 8, 1952

